More than 3000 bush and grass fires have been recorded across the state so far this spring, with 37 burning on Thursday - including a significant blaze through 40 hectares of grassland near Lake Macquarie and which was still burning come night.

''I'd like the public to understand that the people who light fires put everybody at risk,'' Detective Acting Superintendent Ian McNab said in Sydney.

''There's a huge risk in even just a small grass fire. A small grass fire can turn into something catastrophic, as we saw in Victoria back in 2009,'' when the Black Saturday fires killed 173 people and destroyed more than 2000 homes.

The Rural Fire Service deputy commissioner, Rob Rogers, said about 350 bushfires were investigated every year in NSW and about 100 were arson.

Many fires this spring have already been identified as the work of arsonists, including several around St Marys on Wednesday night. Detective Superintendent McNab said arrests in relation to the St Marys fires were expected in coming days.